Efficient batteries are nowadays becoming increasingly important for driving electrically operated vehicles. Such vehicles can be embodied for example as purely electric vehicles, or as vehicles having an electric auxiliary drive, for example in the manner of a hybrid vehicle.
Here, the batteries have to supply ever greater instantaneous powers and additionally have a high storage capacity. In this case, batteries having operating voltages of several hundred volts are nowadays known. Therefore, special high-power batteries, for example lithium-ion batteries, are frequently necessary in order to be able to meet these requirements. In the batteries, it is conventional to connect a plurality of individual battery cells together in parallel and/or in series in order to be able to achieve desired voltages and currents.
In order to improve the efficiency of such high-power batteries and also to achieve a long service life, it is advantageous to cool the cells of the high-power batteries. The prior art contains various cooling concepts, of which some of the most important are mentioned below.
First of all, cooling with water, or with water glycol, is widespread, since this type of cooling is already common in vehicles with conventional drives, i.e. with internal combustion engines. A safety-critical drawback of the use of this cooling medium is the electrical conductivity thereof. In the event of the cooling circuit leaking, for example as a result of an accident, the escaping water or water glycol can cause short circuits. As a result fires and other emergency situations can be caused, and this can in turn lead to additional and sometimes considerable damage to the vehicle. In order to reduce this risk, cooling systems with water or water glycol have in the meantime preferably been embodied such that they cool the battery only outside the battery housing thereof. However, this has the drawback that the heat transfer from the individual cells to the cooling system is slow and local high temperatures can arise in the battery. In particular, a temperature gradient can form in the battery, wherein the cells in the interior of the battery are at a higher temperature than the housing of the battery, on which the cooling system acts. As a result, it is possible for sufficient cooling of the battery not to be able to be ensured in demanding operating situations, with the result that the operational safety of the vehicle is limited.
Alternatively, cooling of the battery with air is known. The heat capacity of air is low compared with water, however, and so a high volumetric flow is required in order to be able to cool the battery sufficiently. This can be ensured only with difficulty and requires sometimes additional components which, on account of the rising costs, their size or of operating noise, are unacceptable.
Likewise, cooling with a particular coolant, for example R1234yf or R134a, is common. These coolants are used for example in air conditioning systems of vehicles. However, these coolants are sometimes difficult to handle and/or not desirable for reasons of environmental protection or operational safety. In addition, these coolants are designed for heat absorption and heat output by evaporation and condensation, this requiring a complicated cooling circuit with corresponding components, i.e. an evaporator and a condenser.
One possibility for cooling the battery consists in using insulating oil as coolant. With this coolant, it is possible in principle to guide the coolant closer to the cells, or even to make the coolant flow directly around the cells. In this way, effective and uniform heat transfer from the individual cells to the coolant can be ensured. However, a problem here is that there is in principle the risk of short circuits of the cells when an insulation fault occurs. Such an insulation fault can occur for example when the coolant is contaminated or in particular water gets into the cooling circuit.
In this connection, DE 10 2013 201 205 A1, which is incorporated by reference herein, discloses a motor-vehicle battery module having one or more battery cells and a coolant loss detection and remedy system which interacts with the coolant and is configured to provide temperature management of the battery module. Coolant levels in or around the battery module can be detected, wherein the interaction with a controller allows a corrective measure if a coolant leak is detected. The controller reads a coolant level sensor which is arranged in a coolant storage tank, in order to establish whether the coolant level drops below a predetermined level. If it is established that the coolant level is low, the controller signals the low level and takes countermeasures. The controller also has a set of activation conditions which have to be met before the controller reads the coolant level sensor.
Furthermore, a deflection-sensitive, coolant-activated system for detecting a drain plug for high-voltage battery packs is known from DE 10 2014 100 008 A1, which is incorporated by reference herein. Accordingly, said document describes a battery pack having a drain plug and a circuit, in order to detect when a liquid coolant has entered the battery pack. The drain plug has a carrier which defines a cavity within the carrier, an inlet which is arranged on a first face of the carrier, and an outlet which is arranged on a second face of the carrier, wherein the first face and the second face are fluidically offset from one another and are coupled to the cavity. A soluble plug can be arranged in the cavity of the carrier; the soluble plug is configured such that it at least partially dissolves when it is fluidically coupled to a coolant, thereby allowing a part of the coolant to flow between the inlet and the outlet and out of the battery pack. A strain gage device interacts with the carrier such that a measured change in resistance in the strain gage device corresponds to the soluble plug in contact with the liquid coolant. An associated circuit with the drain plug notifies the onboard computer systems of the activation of the drain plug.